The study will investigate the role of social structure in population regulatory mechanisms of Microtus ochrogaster and M. pennsylvanicus. Special emphasis will be placed on the relative effects of direct social interactions and pheromones on reproduction. Laboratory experiments will include effects of social contact and pheromones on reproduction within social (family) groups. Small outdoor colonies and confined populations will test results of the laboratory experiments under more natural conditions and refine methodology to be used in field studies. Existence of a basic pheromonal-social interaction system common to diverse taxonomical and ecological species of rodents, with different aspects emphasized in each, depending on evolution of adaptations related to their life-history strategies, is predicted.